Download Principles of Fiber Optic Communication

Principles of Fiber
Optic Communication
Module 4
of
Course 2, Elements of Photonics
OPTICS AND PHOTONICS SERIES
STEP (Scientific and Technological Education
in Photonics), an NSF ATE Project
© 2008 CORD
This document was produced in conjunction with the STEP project—Scientific and
Technological Education in Photonics—an NSF ATE initiative (grant no. 0202424). Any
opinions, findings, and conclusions or recommendations expressed in this material are those of
the author(s) and do not necessarily reflect the views of the National Science Foundation.
For more information about the project, contact either of the following persons:
Dan Hull, PI, Director
CORD
P.O. Box 21689
Waco, TX 76702-1689
(245) 741-8338
(254) 399-6581 fax
[email protected]
Dr. John Souders,
Director of Curriculum Materials
P.O. Box 21689
Waco, TX 76702-1689
(254) 772-8756 ext 393
(254) 772-8972 fax
[email protected]
Published and distributed by
CORD Communications
601 Lake Air Drive, Suite E
Waco, Texas 76710-5841
800-231-3015 or 254-776-1822
Fax 254-776-3906
www.cordcommunications.com
ISBN 1-57837-392-1
PREFACE
This is the fourth module in Course 2 (Elements of Photonics) of the STEP curriculum.
Following are the titles of all six modules in the course:
1.
Operational Characteristics of Lasers
2.
Specific Laser Types
3.
Optical Detectors and Human Vision
4.
Principles of Fiber Optic Communication
5.
Photonic Devices for Imaging, Storage, and Display
6.
Basic Principles and Applications of Holography
The six modules can be used as a unit or independently, as long as prerequisites have been met.
For students who may need assistance with or review of relevant mathematics concepts, a
student review and study guide entitled Mathematics for Photonics Education (available from
CORD) is highly recommended.
The original manuscript of this document was authored by Nick Massa (Springfield Technical
Community College) and edited by Leno Pedrotti (CORD). Formatting and artwork were
provided by Mark Whitney and Kathy Kral (CORD).
CONTENTS
Introduction ...................................................................................................................................................1
Prerequisites ..................................................................................................................................................1
Objectives ......................................................................................................................................................2
Scenario .........................................................................................................................................................3
Basic Concepts ..............................................................................................................................................4
Historical Introduction ..............................................................................................................................4
Benefits of Fiber Optics ............................................................................................................................6
The Basic Fiber Optic Link.......................................................................................................................7
Fiber Optic Cable Fabrication ...................................................................................................................7
Preform fabrication ...............................................................................................................................7
Outside vapor deposition (OVD) ..........................................................................................................9
Inside vapor deposition (IVD) ............................................................................................................10
Vapor axial deposition (VAD)............................................................................................................10
Total Internal Reflection (TIR) ...............................................................................................................11
Transmission Windows...........................................................................................................................12
The Optical Fiber ....................................................................................................................................14
Numerical aperture..............................................................................................................................15
Fiber Optic Loss Calculations.................................................................................................................16
Power budget ......................................................................................................................................19
Types of Optical Fiber ............................................................................................................................21
Step-index multimode fiber ................................................................................................................21
Step-index single-mode fiber..............................................................................................................22
Graded-index fiber..............................................................................................................................23
Polarization-maintaining fiber ............................................................................................................23
Fiber Optic Cable Design........................................................................................................................24
Dispersion ...............................................................................................................................................27
Calculating dispersion.........................................................................................................................28
Intermodal dispersion .........................................................................................................................29
Chromatic dispersion ..........................................................................................................................29
Fiber Optic Sources.................................................................................................................................32
LEDs ...................................................................................................................................................32
Laser diodes ........................................................................................................................................33
Fiber Optic Detectors ..............................................................................................................................34
Connectors ..........................................................................................................................................36
Fiber Optic Couplers ...............................................................................................................................37
Star couplers .......................................................................................................................................37
T-couplers ...........................................................................................................................................39
Wavelength-division multiplexers ......................................................................................................39
Fiber Bragg gratings ...........................................................................................................................41
Erbium-doped fiber amplifiers (EDFA)..............................................................................................42
Fiber Optic Sensors .................................................................................................................................42
Extrinsic fiber optic sensors................................................................................................................43
Intrinsic sensors ..................................................................................................................................45
Laboratory ...................................................................................................................................................48
Problems ......................................................................................................................................................51
References ...................................................................................................................................................52
C OURSE 2: E LEMENTS
OF
P HOTONICS
Module 2-4
Principles of Fiber Optic
Communication
INTRODUCTION
The dramatic reduction of transmission loss in optical fibers coupled with equally important
developments in the area of light sources and detectors have brought about a phenomenal
growth of the fiber optic industry during the past two decades. Its high bandwidth capabilities
and low attenuation characteristics make it ideal for gigabit data transmission and beyond. The
birth of optical fiber communication coincided with the fabrication of low-loss optical fibers
and room-temperature operation of semiconductor lasers in 1970. Ever since, the scientific and
technological progress in this field has been so phenomenal that within a brief span of 30 years
we are already in the fifth generation of optical fiber communication systems. Recent
developments in optical amplifiers and wavelength division multiplexing (WDM) are taking us
to a communication system with almost “zero” loss and “infinite” bandwidth. Indeed, optical
fiber communication systems are fulfilling the increased demand on communication links,
especially with the proliferation of the Internet. In this module, Principles of Fiber Optic
Communication, you will be introduced to the building blocks that make up a fiber optic
communication system. You will learn about the different types of optical fiber and their
applications, light sources and detectors, couplers, splitters, wavelength-division multiplexers,
and other components used in fiber optic communication systems. Non-communications
applications of fiber optics including illumination with coherent light bundles and fiber optic
sensors will also be covered.
PREREQUISITES
Prior to this module, you are expected to have covered Modules 1-1, Nature and Properties of
Light; Module 1-3, Light Sources and Laser Safety; Module 1-4, Basic Geometrical Optics; and
Module 1-5, Basic Physical Optics; Module 1-6, Principles of Lasers; and Module 2-3, Optical
Detectors and Human Vision. In addition, you should be able to manipulate and use algebraic
formulas involving trigonometric functions and deal with units.
1
OBJECTIVES
Upon completion of this module, you will be able to:
2
•
Identify the basic components of a fiber optic communication system
•
Discuss light propagation in an optical fiber
•
Identify the various types of optical fibers
•
Discuss the dispersion characteristics for various types of optical fibers
•
Identify selected types of fiber optic connectors
•
Calculate numerical aperture (N.A.), intermodal dispersion, and material dispersion.
•
Calculate decibel and dBm power
•
Calculate the power budget for a fiber optic system
•
Calculate the bandwidth of an optical fiber
•
Describe the operation and applications of fiber optic couplers
•
Discuss the differences between LEDs and laser diodes with respect to performance
characteristics
•
Discuss the performance characteristics of optical detectors
•
Discuss the principles of wavelength-division multiplexing (WDM)
•
Discuss the significance of the International Telecom Union grid (ITU grid)
•
Discuss the use of erbium-doped fiber amplifiers (EDFA) for signal regeneration
•
Describe the operation and applications of fiber Bragg gratings
•
Describe the operation and application of fiber optic circulators
•
Describe the operation and application of fiber optic sensors.
Optics and Photonics Series, Course 2: Elements of Photonics
SCENARIO
Dante is about to complete a bachelor’s degree in fiber optic technology, a field that has
interested him since high school. To prepare himself for the highly rewarding careers that fiber
optics offers, Dante took plenty of math and science in high school and then enrolled in an
associate degree program in laser electro-optics technology at Springfield Technical
Community College (STCC) in Springfield, Massachusetts. Upon graduation from STCC he
accepted a position as an electro-optics technician at JDS Uniphase Corporation in Bloomfield,
Connecticut. The company focuses on precision manufacturing of the high-speed fiber optic
modulators and components that are used in transmitters for the telecommunication and cable
television industry.
As a technician at JDS Uniphase, Dante was required not only to understand how fiber optic
devices work but also to have an appreciation for the complex manufacturing processes that are
required to fabricate the devices. The background in optics, fiber optics, and electronics that
Dante received at STCC proved to be invaluable in his day-to-day activities. On the job, Dante
routinely worked with fusion splicers, optical power meters, and laser sources and detectors, as
well as with optical spectrum analyzers and other sophisticated electronic test equipment.
After a few years as an electro-optics technician, Dante went on to pursue a bachelor’s degree in
fiber optics. (The courses he had taken at STCC transferred, so he was able to enroll in his
bachelor’s program as a junior.) Because of his hard work on the job at JDS Uniphase, Dante
was awarded a full scholarship and an internship at JDS Uniphase. This allowed Dante to
complete his degree while working for JDS Uniphase part time. According to Dante, “the
experience of working in a high-tech environment while going to school really helps you see the
practical applications of what you are learning—which is especially important in a rapidly
changing field like fiber optics.”
Module 2-4: Principles of Fiber Optic Communication
3
BASIC CONCEPTS
Historical Introduction
Communication implies transfer of information from one point to another. When it is necessary
to transmit information, such as speech, images, or data, over a distance, one generally uses the
concept of carrier wave communication. In such a system, the information to be sent modulates
an electromagnetic wave such as a radio wave, microwave, or light wave, which acts as a
carrier. (Modulation means to vary the amplitude or frequency in accordance with an external
signal.) This modulated wave is then transmitted to the receiver through a channel and the
receiver demodulates it to retrieve the imprinted signal. The carrier frequencies associated with
TV broadcast (∼ 50–900 MHz) are much higher than those associated with AM radio broadcast
(∼ 600 kHz–20 MHz). This is due to the fact that, in any communication system employing
electromagnetic waves as the carrier, the amount of information that can be sent increases as the
frequency of the carrier is increased.1 Obviously, TV broadcast has to carry much more
information than AM broadcasts. Since optical beams have frequencies in the range of
1014 to 1015 Hz, the use of such beams as the carrier would imply a tremendously large increase
in the information-transmission capacity of the system as compared to systems employing radio
waves or microwaves.
In a conventional telephone system, voice signals are converted into equivalent electrical
signals by the microphone and are transmitted as electrical currents through metallic (copper or
aluminum) wires to the local telephone exchange. Thereafter, these signals continue to travel as
electric currents through metallic wire cable (or for long-distance transmission as
radio/microwaves to another telephone exchange) usually with several repeaters in between.
From the local area telephone exchange, at the receiving end, these signals travel via metallic
wire pairs to the receiver telephone, where they are converted back into corresponding sound
waves. Through such cabled wire-pair telecommunication systems, one can at most send
48 simultaneous telephone conversations intelligibly. On the other hand, in an optical
communication system that uses glass fibers as the transmission medium and light waves as
carrier waves, it is distinctly possible today to have 130,000 or more simultaneous telephone
conversations (equivalent to a transmission speed of about 10 Gbit/s) through one glass fiber no
thicker than a human hair. This large information-carrying capacity of a light beam is what
generated interest among communication engineers and caused them to explore the possibility
of developing a communication system using light waves as carrier waves.
The idea of using light waves for communication can be traced as far back as 1880 when
Alexander Graham Bell invented the photophone (see Figure 4-1) shortly after he invented the
telephone in 1876. In this remarkable experiment, speech was transmitted by modulating a light
beam, which traveled through air to the receiver. The flexible reflecting diaphragm (which
1
The information-carrying capacity of an electromagnetic carrier is approximately proportional to the difference
between the maximum and the minimum frequencies (technically known as bandwidth of the channel) that can be
transmitted through the communication channel. The higher one goes in the electromagnetic spectrum in frequency
scale, the higher the bandwidth and hence the information-carrying capacity of such a communication system. That
is why historically the trend in carrier wave communication has been always toward bandwidths of higher and
higher frequencies.
4
Optics and Photonics Series, Course 2: Elements of Photonics
could be activated by sound) was illuminated by sunlight. A parabolic reflector placed at a
distance of about 200 m received the reflected light. The parabolic reflector concentrated the
light on a photoconducting selenium cell, which formed a part of a circuit with a battery and a
receiving earphone. Sound waves present in the vicinity of the diaphragm vibrated the
diaphragm, which led to a consequent variation of the light reflected by the diaphragm. The
variation of the light falling on the selenium cell changed the electrical conductivity of the cell,
which in turn changed the current in the electrical circuit. This changing current reproduced the
sound on the earphone.
Figure 4-1 Schematic of the photophone invented by Bell. In this system, sunlight was modulated by a
vibrating diaphragm and transmitted through a distance of about 200 meters in air to a receiver
containing a selenium cell connected to the earphone.
After succeeding in transmitting a voice signal over 200 meters using a light signal, Bell wrote
to his father: “I have heard a ray of light laugh and sing. We may talk by light to any visible
distance without any conducting wire.” To quote from MacLean: “In 1880 he [Graham Bell]
produced his ‘photophone’ which to the end of his life, he insisted was ‘…. the greatest
invention I have ever made, greater than the telephone…’ Unlike the telephone, though, it had
no commercial value.”
The modern impetus for telecommunication with carrier waves at optical frequencies owes its
origin to the discovery of the laser in 1960. Earlier, no suitable light source was available that
could reliably be used as the information carrier.2 At around the same time, telecommunication
traffic was growing very rapidly. It was conceivable then that conventional telecommunication
systems based on coaxial cables, radio and microwave links, and wire-pair cable, could soon
reach a saturation point. The advent of lasers immediately triggered a great deal of investigation
aimed at examining the possibility of building optical analogues of conventional communication
systems. The very first such modern optical communication experiment involved laser beam
transmission through the atmosphere. However, it was soon realized that shorter-wavelength
laser beams could not be sent in open atmosphere through reasonably long distances to carry
signals, unlike, for example, the longer-wavelength microwave or radio systems. This is due to
2
We may mention here that, although incoherent sources like light-emitting diodes (LED) are also often used in
present-day optical communication systems, it was the discovery of the laser that triggered serious interest in the
development of optical communication systems.
Module 2-4: Principles of Fiber Optic Communication
5
the fact that a laser light beam (of wavelength about 1 µm) is severely attenuated and distorted
owing to scattering and absorption by the atmosphere. Thus, for reliable light-wave
communication under terrestrial environments it would be necessary to provide a “guiding”
medium that could protect the signal-carrying light beam from the vagaries of the terrestrial
atmosphere. This guiding medium is the optical fiber, a hair-thin structure that guides the light
beam from one place to another through the process of total internal reflection (TIR), which
will be discussed in the next section.
Benefits of Fiber Optics
Fiber optic communication systems have many advantages over copper wire-based
communication systems. These advantages include:
•
Long-distance signal transmission
The low attenuation and superior signal quality of fiber optic communication systems allow
communications signals to be transmitted over much longer distances than metallic-based
systems without signal regeneration. In 1970, Kapron, Keck, and Maurer (at Corning Glass
in USA) were successful in producing silica fibers with a loss of about 17 dB/km at a
wavelength of 633 nm. Since then, the technology has advanced with tremendous rapidity.
By 1985 glass fibers were routinely produced with extremely low losses (< 0.2 dB/km).
Voice-grade copper systems require in-line signal regeneration every one to two kilometers.
In contrast, it is not unusual for communications signals in fiber optic systems to travel over
100 kilometers (km), or about 62 miles, without signal amplification of regeneration.
•
Large bandwidth, light weight, and small diameter
Today’s applications require an ever-increasing amount of bandwidth. Consequently, it is
important to consider the space constraints of many end users. It is commonplace to install
new cabling within existing duct systems or conduit. The relatively small diameter and light
weight of optical cable make such installations easy and practical, saving valuable conduit
space in these environments.
•
Nonconductive
Another advantage of optical fibers is their dielectric nature. Since optical fiber has no
metallic components, it can be installed in areas with electromagnetic interference (EMI),
including radio frequency interference (RFI). Areas with high EMI include utility lines,
power-carrying lines, and railroad tracks. All-dielectric cables are also ideal for areas of
high lightning-strike incidence.
•
Security
Unlike metallic-based systems, the dielectric nature of optical fiber makes it impossible to
remotely detect the signal being transmitted within the cable. The only way to do so is by
accessing the optical fiber. Accessing the fiber requires intervention that is easily detectable
by security surveillance. These circumstances make fiber extremely attractive to
governmental bodies, banks, and others with major security concerns.
6
Optics and Photonics Series, Course 2: Elements of Photonics
The Basic Fiber Optic Link
Figure 4-2 shows a typical optical fiber communication system. It consists of a transmitting
device T that converts an electrical signal into a light signal, an optical fiber cable that carries
the light, and a receiver R that accepts the light signal and converts it back into an electrical
signal. The complexity of a fiber optic system can range from very simple (i.e., local area
network) to extremely sophisticated and expensive (i.e., long-distance telephone or cable
television trunking). For example, the system could be built very inexpensively using a visible
LED, plastic fiber, a silicon photodetector, and some simple electronic circuitry.
On the other hand, a system used for long-distance, high-bandwidth telecommunication that
employs wavelength-division multiplexing, erbium-doped fiber amplifiers, external modulation
using distributed feedback (DFB) lasers with temperature compensation, fiber Bragg gratings,
and high-speed infrared photodetectors can be very expensive. The basic question is how much
information is to be sent and how far does it have to go? With this in mind we will first examine
the basic principles of fiber optics. We will then move on to the various components that make
up a fiber optic communication system, and finally look at the considerations that must be taken
into account in the design of a simple fiber optic link
Figure 4-2 A typical fiber optic communication system: T, transmitter; C, connector; S, splice;
R, repeater; D, detector, and coils of fibers
Fiber Optic Cable Fabrication
The fabrication of fiber optic cable consists of two processes: preform fabrication and fiber
draw. Preform fabrication involves manufacturing a glass “perform” consisting of a core and
cladding with the desired index profile of the fiber. Fiber draw involves heating the preform to
about 2000° C and drawing it down to the desired diameter and adding a protective buffer
coating.
Preform fabrication
The fabrication process for creating the glass preform (Figure 4-3) from which fiber optic cable
is drawn involves forming a glass rod that has the desired index profile and core/cladding
dimension ratio. This process, known as chemical vapor deposition or CVD, was developed by
Corning scientists in the 1970’s and has made it possible to create ultra-pure glass fiber suitable
for optical transmission over very long distances. Using the CVD method, the ultra-pure glass
that makes up the preform is synthesized from ultra-pure liquid or gaseous reactants, typically,
silicon chloride (SiCl4), germanium chloride (GeCl4), oxygen, and hydrogen. This reaction
Module 2-4: Principles of Fiber Optic Communication
7
produces a very fine “soot” of silicon and germanium oxide, which is then vitrified forming
ultra-pure glass.
Figure 4-3 Two views of fiber optic preform fabrication (Sources: Upper—Fibercore Limited of
Chilworth UK, a wholly-owned subsidiary of Scientific Atlanta Inc. of Lawrenceville, Georgia; used by
permission. Lower—OFS; used by permission)
There are three processes commonly used to manufacture glass preforms:
1. Outside vapor deposition (OVD): Silicon and germanium particles are deposited on the
surface of a rotating target rod.
2. Inside vapor deposition (IVD): A soot consisting of silicon and germanium particles is
deposited on the inside walls of hollow glass tube.
3. Axial vapor deposition (AVD): Deposition is done axially, directly in the glass preform.
Inside vapor deposition (IVD) and outside vapor deposition (OVD) require a collapse stage to
close the hollow gap in the center of the preform after the soot is deposited. Outside vapor
8
Optics and Photonics Series, Course 2: Elements of Photonics
deposition (OVD) and axial vapor deposition (AVD) require sintering to vitrify the soot after
they have been deposited.
Outside vapor deposition (OVD)
The OVD process for manufacturing optical fiber typically consists of three stages:
1. Laydown – Depositing the glass soot which will eventually form the glass preform
2. Consolidation – Heating the glass soot in a furnace to solidify the glass preform
3. Draw – Heating up the glass preform and drawing the glass into a fine strand of fiber
In the laydown stage (see Figure 4-4), many fine layers of silicon and germanium soot are
deposited onto a ceramic rod. During the laydown stage, SiCl4 and GeCl4 vapors are passed over
the rotating rod and react with oxygen to generate SiO2 and GeO2. A traversing burner flame
forms fine soot particles of silica and germania on the rod forming the core and cladding layers
of the fiber. The GeCl4 serves as a dopant to increase the index of refraction of the core.
Figure 4-4 Outside vapor deposition
The OVD process is distinguished by the method of depositing the soot on the ceramic rod. The
core material is deposited first, followed by the cladding material. Since the core and cladding
materials are deposited using vapor deposition, the entire resulting preform is extremely pure.
When the deposition process is complete, the ceramic rod is removed from the center of the
porous preform and the hollow preform is placed into a consolidation furnace. During the high
temperature consolidation process, water vapor is removed from the preform and sintering
condenses the preform into a solid, dense, transparent rod.
Module 2-4: Principles of Fiber Optic Communication
9
During the draw process (see Figure 4-5), the
finished glass preform is placed in a draw
tower and drawn into a single continuous
strand of glass fiber. A draw tower consists of
a furnace to heat up the glass preform into
molten glass, a diameter-measuring device
(typically a laser micrometer), a coating
chamber for applying a protective coating, and
a take-up spool for winding the finished fiber.
A typical draw tower can be several stories
tall. The glass preform is first lowered into the
draw furnace. The tip of the preform is then
heated to about 2000° C until a piece of
molten glass (called a “gob”), begins to fall
due to the force of gravity. When the gob falls,
it pulls behind it a fine glass fiber and cools. A
draw tower operator then cuts off the gob and
threads the fine fiber strand into a tractor
assembly. The tractor assembly speeds up or
slows down to provide tension to the fiber
Figure 4-5 Fiber draw process
stand, which controls the diameter of the fiber.
The laser-based diameter monitor measures the diameter of the fiber hundreds of times per
second to ensure that the outside diameter of the fiber is held to acceptable tolerance levels
(typically ±1 um). As the fiber is drawn, a protective coating is applied and cured using UV
light.
Inside vapor deposition (IVD)
Inside vapor deposition uses a process
known as modified chemical vapor
deposition (MCVD) to deposit the soot
on the inside walls of a tube of ultrapure silica. (See Figure 4-6.) In this
method, a tube of ultra-pure silica is
mounted on a glass-working lathe,
equipped with an oxygen-hydrogen
burner. The chlorides and oxygen are
introduced from one end of the tube,
and caused to react by the heat of the
burner. The resulting soot (submicron
Figure 4-6 Inside vapor deposition
particles of silica and germania) is
deposited inside the tube through a phenomenon known as thermophoresis. As the burner passes
over the deposits, they are vitrified into solid glass. By varying the ratio of silicon and
germanium chloride, the refractive index profile is built layer after layer, from the outside to the
core. The more germanium, the higher the refractive index of the glass. When the deposition
process is complete, the preform is heated to collapse the hollow tube into a solid preform.
10
Optics and Photonics Series, Course 2: Elements of Photonics
Vapor axial deposition (VAD)
The vapor axial deposition process involves the deposition of glass soot on the end of a rotating
pure silica boule. (See Figure 4-7.) The initial soot deposit forms the core of the preform.
Additional layers of soot are then added radially outward until the final desired refractive index
profile is achieved. The benefit of vapor axial deposition is that no hole is created. This
eliminates the need for both a central ceramic rod (as in OVD) and the need to collapse the
preform to eliminate the hole as in IVD.
Figure 4-7 Vapor axial deposition
Total Internal Reflection (TIR)
At the heart of an optical communication system is the optical fiber that acts as the transmission
channel carrying the light beam loaded with information. As mentioned earlier, the guidance of
the light beam (through the optical fiber) takes place because of the phenomenon of total
internal reflection (TIR), which we will now discuss. You learned about critical angles, TIR,
etc. in Module 1-4, Basic Geometrical Optics. You need now to refresh your memory and apply
these ideas more directly to the physics of optical fibers. We first define the refractive index (n)
of a medium:
n = vc
(4-1)
where c (≈ 3 × 108 m/s) is the speed of light in free space and v represents the velocity of light
in that medium. For example, for light waves, n ≈ 1.5 for glass and n ≈ 1.33 for water.
Figure 4-8 (a) A ray of light incident on a denser medium (n2 > n1). (b) A ray incident on a rarer
medium (n2 < n1). (c) For n2 < n1, if the angle of incidence is greater than the critical angle, the incident
ray will undergo total internal reflection.
Module 2-4: Principles of Fiber Optic Communication
11
As you know, when a ray of light is incident at the interface of two media (like air and glass),
the ray undergoes partial reflection and partial refraction as shown in Figure 4-8a. The vertical
dotted line represents the normal to the surface. The angles φ1, φ2, and φr represent the angles
that the incident ray, refracted ray, and reflected ray make with the normal. According to Snell’s
law and the law of reflection,
n1 sin φ1 = n2 sin φ2 (Snell’s law)
φ1 = φr (Law of reflection)
(4-2)
Further, the incident ray, reflected ray, and refracted ray lie in the same plane. In Figure 4-8a,
we know from Snell’s law that since n2 > n1, we must have φ2 < φ1 (i.e., the refracted ray will
bend toward the normal). On the other hand, if a ray is incident at the interface of a medium
where n2 < n1, the refracted ray will bend away from the normal (see Figure 4-8b). The angle of
incidence, for which the angle of refraction is 90°, is known as the critical angle and is denoted
by φc. Thus, when
⎛ n2 ⎞
⎟⎟
⎝ n1 ⎠
φ1 = φc = sin –1 ⎜⎜
(4-3)
the angle of incidence exceeds the critical angle (i.e., when φ1 > φc), there is no refracted ray
and we have total internal reflection TIR. (See Figure 4-8c and Figure 4-10b).
Example 1
For a glass-air interface, n1 = 1.5, n2 = 1.0, and the critical angle is given by
φc = sin–1 (1.0/1.5) ≈ 41.8°
On the other hand, for a glass-water interface, n1 = 1.5, n2 = 1.33, and
φc = sin–1 (1.33/1.5) ≈ 62.5°.
Transmission Windows
Optical fiber communication systems transmit information at wavelengths that are in the nearinfrared portion of the spectrum, just above the visible, and thus undetectable to the unaided
eye. Typical optical transmission wavelengths are 850 nm, 1310 nm, and 1550 nm. Both lasers
and LEDs are used to transmit light through optical fiber. Lasers are usually used primarily for
1310 and 1550-nm single-mode applications. LEDs are used for 850 nm multimode
applications.
12
Optics and Photonics Series, Course 2: Elements of Photonics
Figure 4-9 Typical wavelength dependence of attenuation for a silica fiber. Notice that the lowest
attenuation occurs at 1550 nm [adapted from Miya, Hasaka, and Miyashita].
Figure 4-9 shows the spectral dependence of fiber attenuation (i.e., dB loss per unit length) as a
function of wavelength of a typical silica optical fiber. The losses are caused by various
mechanisms such as Rayleigh scattering, absorption due to metallic impurities and water in the
fiber, and intrinsic absorption by the silica molecule itself. The Rayleigh scattering loss varies
as 1/λ04, i.e., longer wavelengths scatter less than shorter wavelengths. (Here λ0 represents the
free space wavelength.) As we can see in Figure 4-9, Rayleigh scatter causes the dB loss/km to
decrease gradually as the wavelength increases from 800 nm to 1550 nm. The two absorption
–
peaks around 1240 nm and 1380 nm are primarily due to traces of OH ions and metallic ions in
the fiber. For example, even 1 part per million (ppm) of iron can cause a loss of about
–
0.68 dB/km at 1100 nm. Similarly, a concentration of 1 ppm of OH ion can cause a loss of 4
dB/km at 1380 nm. This shows the level of purity that is required to achieve low-loss optical
fibers. If these impurities are removed, the two absorption peaks will disappear. For
λ0 > 1600 nm, the increase in the dB/km loss is due to the absorption of infrared light by silica
molecules. This is an intrinsic property of silica, so no amount of purification can remove this
infrared absorption tail.
As you see, there are two windows at which the dB/km loss attains its minimum value. The first
window is around 1300 nm (with a typical loss coefficient of less than 1 dB/km) where,
fortunately (as we will see later), the material dispersion is negligible. However, the loss
coefficient is at its absolute minimum value of about 0.2 dB/km around 1550 nm. The latter
window has become extremely important in view of the availability of erbium-doped fiber
amplifiers.
Module 2-4: Principles of Fiber Optic Communication
13
The Optical Fiber
An optical fiber (Figure 4-10) consists of a central glass core of radius “a” surrounded by an
outer cladding made of glass with a slightly lower refractive index. The corresponding
refractive index distribution (in the transverse direction) is given by:
n = n1
for
r<a
n = n2
for
r>a
(4-4)
Figure 4-10 (a) A glass fiber consists of a cylindrical central core surrounded by a cladding material of
slightly lower refractive index. (b) Light rays impinging on the core-cladding interface at an angle φ
greater than the critical angle φc are trapped inside the core of the fiber and reflected back and forth (A,
C, B, etc.) along the core-cladding interface.
Figure 4-10 shows a light ray incident on the air-core left interface at an angle θi. The ray
refracts at angle θ in accordance with Snell’s law and then strikes the core-cladding interface at
angle φ. In the drawing shown, the angle φ is greater than the critical angle φc defined in
Equation 4-3, thus leading to total internal reflection at A. The reflected ray is totally internally
reflected again at C and B and so on, remaining trapped in the fiber as it propagates along the
core axis.
The core diameter d = 2a of a typical telecommunication-grade multimode fiber is
approximately 62.5 µm with an outer cladding diameter of 125 um. The cladding index
n2 is approximately 1.45 (pure silica), and the core index n1, barely larger, around 1.465. The
cladding is usually pure silica while the core is usually silica doped with germanium, which
increases the refractive index slightly from n2 to n1. The core and cladding are fused together
during the manufacturing process and typically not separable. An outside plastic buffer is
usually added to protect the fiber from environmental contaminants.
14
Optics and Photonics Series, Course 2: Elements of Photonics
Numerical aperture
One of the more important parameters associated with fiber optics is the numerical aperture.
The numerical aperture of a fiber is a measure of its light-gathering ability and is defined by
N.A. = Sin (θa)max
(4-5)
where (θa)max is the maximum half-acceptance angle of the fiber, as shown in Figure 4-11.
Figure 4-11 Numerical aperture
The larger the numerical aperture, the greater the light gathering ability of the fiber. Typical
values for N.A. are between 0.2 and 0.3 for multimode fiber and 0.1 to 0.2 for single-mode
fiber. The numerical aperture is an important quantity because it is used to determine the
coupling and dispersion characteristics of a fiber. For example, a large numerical aperture
allows for more light to be coupled into the fiber but at the expense of modal dispersion, which
causes pulse spreading and ultimately bandwidth limitations.
The numerical aperture (N.A.) is related to the index of refraction of the core and cladding by
the following equation:
N.A. = sin(θa ) max = n 21 − n 2 2
(4-6)
As can be seen, the larger the difference between the core and cladding index, the larger the
numerical aperture and hence more modal dispersion.
The N.A. may also be expressed in terms of the relative refractive index difference termed ∆,
where
( n12 − n2 2 )
∆≡
2n12
(4-7)
so that, with Equation 4-6, we get Equation 4-8.
∆=
(N.A.)2
2n12
Module 2-4: Principles of Fiber Optic Communication
(4-8)
15
Combining Equations 4-6 and 4-8, we obtain a useful relation, Equation 4-9.
N.A. = sin(θa )max = n1 2∆
(4-9)
In short, a large N.A. represents a large difference in refractive index, leading to a large
acceptance angle and hence a large numerical aperture. However, this can lead to serious
bandwidth limitations. Typical values for ∆ range from 0.01 to 0.03 or 1 to 3 %
Example 2
For a typical step-index (multimode) fiber with core index n1 ≈ 1.45 and ∆ ≈ 0.01, we get
sin(θa)max = n1 2∆ = 1.45 2 × (0.01) = 0.205
so that (θa)max ≈ 12°. Thus, all light entering the fiber must be within a cone of half-angle 12°. The
full acceptance angle is 2 × 12° = 24°.
Fiber Optic Loss Calculations
Loss in a fiber optic system is expressed in terms of the optical power available at the output
end with respect to the optical power at the input. As follows:
Loss =
Pout
Pin
(4-10)
where Pin is the input power to the fiber and Pout is the power available at the output end of the
fiber. For convenience, fiber optic loss is often expressed in terms of decibels (dB) where
LossdB = 10 log
Pout
Pin
(4-11)
Fiber optic cable manufacturers usually specify loss in optical fiber in terms of decibels per
kilometer (dB/km), as discussed earlier in connection with Figure 4-9.
Example 3
A fiber of 50-km length has Pin = 10 mW and Pout = 1 mW. Find the loss in dB/km.
From Equation 4-11
⎡ 1 mW ⎤
Loss dB = 10log ⎢
⎥ = −10dB (The negative sign indicates a loss.)
⎣10 mW ⎦
And so the loss per unit length of fiber, dB/km, is equal to
Loss(dB/km) = (–10 dB/50 km) = –0.2 dB/km
16
Optics and Photonics Series, Course 2: Elements of Photonics
Example 4
A 10-km fiber optic communication system link has a fiber loss of 0.30 dB/km. Find the output
power if the input power is 20 mW.
Solution
From Equation 4-11, making use of the relationship that y = 10 x if x = log y,
⎛P ⎞
Loss dB = 10 log ⎜ out ⎟
⎝ Pin ⎠
⎛P ⎞
Loss dB
= log ⎜ out ⎟
10
⎝ Pin ⎠
which becomes, then,
10
LossdB
10
⎛ Pout ⎞
⎟.
⎝ Pin ⎠
= ⎜
So, finally, we have
Pout = Pin × 10
LossdB
10
(4-12)
For fiber with a 0.30-dB/km loss characteristic, the lossdB for 10 km of fiber becomes
LossdB = 10 km × (–0.30 dB/km) = –3 dB
Plugging this back into Equation 4-12 we get,
−3
Pout = 20 mW × 10 10 = 10 mW
Optical power in fiber optic systems is often expressed in terms of dBm, a decibel term that
references power to a 1 mWatt (milliwatt) input power level. Optical power here can refer to the
power of a laser source or just to the power somewhere in the system. If Pin in Equation 4-11 is
given as 1 milliwatt, then the power in dBm can be determined using equation 4-13:
⎛ P ⎞
P(dBm) = 10 log ⎜ out ⎟
(4-13)
⎝ 1 mW ⎠
With optical power expressed in dBm, output power anywhere in the system can be determined
simply by expressing the input power in dBm and then subtracting the individual component
losses, also expressed in dB. It is important to note that an optical source with a power input of
1 mW can be expressed as 0 dBm, as indicated by Equation 4-13, since
⎛ 1 mW ⎞
10log ⎜
⎟ = 10log(1) = (10)(0) = 0.
⎝ 1 mW ⎠
The use of decibels provided a convenient method of expressing optical power in fiber optic
systems. For example, for every 3 dB loss in optical power, the power in milliwatts is cut in
half. Consequently, for every 3-dB increase in optical power, the optical power in milliwatts is
doubled. For example, a 3-dBm optical source has a P of 2 mW, whereas a –6-dBm source has a
Module 2-4: Principles of Fiber Optic Communication
17
P of 0.25 mW, as can be verified with Equation 4-13. Furthermore, every increase or decrease
of 10 dB in optical power corresponds to a 10-fold increase or decrease in optical power in
expressed in milliwatts. For example, whereas 0 dBm corresponds to 1-milliwatt of optical
power, 10 dBm would be 10 milliwatts, and 20 dBm would be 100 milliwatts. Similarly,
–10 dBm corresponds to 0.1 milliwatt, and –20 dBm would be 0.01 milliwatts, etc.
Example 5
A 3-km fiber optic system has an input power of 2 mW and a loss characteristic of 2 dB/km.
Determine the output power of the fiber optic system in mW.
Solution
Using Equation 4-13, we convert the source power of 2 mW to its equivalent in dBm:
⎛ 2 mW ⎞
Input powerdBm = 10log ⎜
⎟ = +3 dBm
⎝ 1 mW ⎠
The lossdB for the 3-km cable is,
LossdB = 3 km × 2 dB/km = 6 dB
Thus, power in dB is
(Output power)dB = +3 dBm – 6 dB = –3 dBm
Using Equation 4-13 to convert the output power of –3 dBm back to milliwatts, we have
P (dBm) = 10 log
or
P (mW)
1 mW
P(dBm)
P (mW)
= log
,
10
1 mW
P (dBm)
P(mW)
= 10 10
or
1(mW)
so that
P (mW) = 1 mW × 10
P (dBm)
10
Plugging in for P(dBm) = –3 dBm, we get for the output power in milliwatts
–3
Pout (mW) = 1 mW × 10 10 = 0.5 mW
Note that one can also use Equation 4-12 to get the same result, where now Pin = 2 mW and
LossdB = –6 dB:
Pout = Pin
Loss
dB
× 10 10
–6
or Pout =2 mW × 10 10 = 0.5 mW, the same as above.
18
Optics and Photonics Series, Course 2: Elements of Photonics
Power budget
When designing a fiber optic communication system, one of the main factors that must be
considered is whether or not there is enough power available at the receiver to detect the
transmitted signal after all of the system losses have been accounted for. The process for
accounting for all of the system losses is called a power budget.
The power arriving at the detector must be sufficient to allow clean detection with few errors.
Clearly, the signal at the receiver must be larger than the noise level. The power at the detector,
Pr, must be above the threshold level or receiver sensitivity Ps.
Pr ≥ Ps
(4-14)
The receiver sensitivity Ps is the signal power, in dBm, at the receiver that results in a particular
bit error rate (BER). Typically the BER is chosen to be at most one error in 1012 bits or 10–12.
Example 6
A receiver has sensitivity Ps of – 45 dBm for a BER of 10–12. What is the minimum power that must
be incident on the detector?
Solution
Use Equation 4-13 to find the source power in milliwatts, given the power sensitivity in dBm. Thus,
⎛ P ⎞
⎟
⎝ 1 mW ⎠
– 45 dBm = 10 log ⎜
or
−45 dBm
P
= 10 10 ,
1 mW
so that
P = (1 mW) × 10–4.5 = 3.16 × 10–5 mW = 31.6 microwatts
for a probability of error of 1 in 1012.
The received power at the detector is a function of:
1. Power emanating from the light source (PL)
2. Source-to-fiber loss (Lsf)
3. Fiber loss per km (FL) for a length of fiber (L)
4. Connector or splice losses (Lconn)
5. Fiber-to-detector loss (Lfd)
The power margin is the difference between the received power Pr and the receiver sensitivity Ps
by some margin Lm.
Lm = Pr – Ps
(4-15)
where Lm is the loss margin in dB, Pr is the received power, and Ps is the receiver sensitivity in
dBm.
Module 2-4: Principles of Fiber Optic Communication
19
If all of the loss mechanisms in the system are taken into consideration, the loss margin can be
expressed as Equation 4-16.
Lm = PL – Lsf – (FL × L) – Lconn – Lfd – Ps
(4-16)
All units are in dB and dBm.
Example 7
A system has the following characteristics:
Source power (PL) = 2 mW (3 dBm)
Source to fiber loss (Lsf) = 3 dB
Fiber loss per km (FL) = 0.5 dB/km
Fiber length (L) = 40 km
Connector loss (Lconn) = 1 dB (one connector between two 20-m fiber lengths)
Fiber to detector loss (Lfd) = 3 dB
Receiver sensitivity (Ps) = –36 dBm
Find the loss margin Lm.
Solution
Lm = 3 dBm – 3 dB – (40 km × 0.5 dB/km) – 1 dB – 3 dB – (–36 dBm) = 12 dB
This particular fiber optic loss budget is illustrated in Figure 4-12, with each loss graphically
depicted.
Figure 4-12 Fiber optic loss budget
20
Optics and Photonics Series, Course 2: Elements of Photonics
Types of Optical Fiber
There are three basic types of fiber optic cable used in communication systems: step-index
multimode, step-index single-mode, and graded-index multimode. These are illustrated in
Figure 4-13.
Figure 4-13 Types of fiber
Step-index multimode fiber
Step-index multimode fiber has an index of refraction profile that “steps” from low-to-high-tolow as measured from cladding-to-core-to-cladding. A relatively large core diameter 2a and
numerical aperture N.A. characterize this fiber. The core/cladding diameter of a typical
multimode fiber used for telecommunication is 62.5/125 µm (about the size of a human hair).
The term “multimode” refers to the fact that multiple modes or paths through the fiber are
possible, as indicated in Figure 4-13a. Step-index multimode fiber is used in applications that
require high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area
network or a campus network backbone.
The major benefits of multimode fiber are: (1) it is relatively easy to work with; (2) because of
its larger core size, light is easily coupled to and from it; (3) it can be used with both lasers and
LEDs as sources; and (4) coupling losses are less than those of the single-mode fiber. The
drawback is that because many modes are allowed to propagate (a function of core diameter,
wavelength, and numerical aperture) it suffers from intermodal dispersion, which will be
discussed in the next section. Intermodal dispersion limits bandwidth, which translates into
lower data rates.
Module 2-4: Principles of Fiber Optic Communication
21
Step-index single-mode fiber
Single-mode step-index fiber (Figure 4-13b) allows for only one path, or mode, for light to
travel within the fiber. In a multimode step-index fiber, the number of modes Mn propagating
can be approximated by
V2
Mn =
(4-17)
2
Here V is known as the normalized frequency, or the V-number, which relates the fiber size, the
refractive index, and the wavelength. The V-number is given by Equation 4-18.
⎡ 2πa ⎤
V =⎢
× N.A.
⎣ λ ⎥⎦
(4-18)
⎡ 2πa ⎤
V =⎢
⎥ × n1 2∆
⎣ λ ⎦
(4-19)
or by Equation 4-19.
In either equation, a is the fiber core radius, λ is the operating wavelength, N.A. is the
numerical aperture, n1 is the core index, and ∆ is the relative refractive index difference between
core and cladding.
The analysis of how the V-number is derived is beyond the scope of this module. But it can be
shown that by reducing the diameter of the fiber to a point at which the V-number is less than
2.405, higher-order modes are effectively extinguished and single-mode operation is possible.
Example 8
What is the maximum core diameter for a fiber if it is to operate in single mode at a wavelength of
1550 nm if the N.A. is 0.12?
From Equation 4-18,
⎡ 2πa ⎤
× N.A.
V =⎢
⎣ λ ⎥⎦
Solving for a yields
a=
Vλ
2π(N.A.)
For single-mode operation, V must be 2.405 or less. The maximum core diameter occurs when
V = 2.405. So, plugging into the equation, we get
(2.405)(1550 nm)
amax =
= 4946 × 10–9 m = 4.95 µm
(2π)(0.12)
or, dmax = 2 × a = 9.9 µm
The core diameter for a typical single-mode fiber is between 5 and 10 µm with a 125-µm
cladding. Single-mode fibers are used in applications such as long distance telephone lines, widearea networks (WANs), and cable TV distribution networks where low signal loss and high data
22
Optics and Photonics Series, Course 2: Elements of Photonics
rates are required and repeater/amplifier spacing must be maximized. Because single-mode fiber
allows only one mode or ray to propagate (the lowest-order mode), it does not suffer from
intermodal dispersion like multimode fiber and therefore can be used for higher bandwidth
applications. At higher data rates, however, single-mode fiber is affected by chromatic dispersion,
which causes pulse spreading due to the wavelength dependence on the index of refraction of
glass (to be discussed in more detail in the next section). Chromatic dispersion can be overcome
by transmitting at a wavelength at which glass has a fairly constant index of refraction (~1300 nm)
or by using an optical source such as a distributed-feedback laser (DFB laser) that has a very
narrow output spectrum. The major drawback of single-mode fiber is that compared to step-index
multimode fiber, it is relatively difficult to work with (i.e., splicing and termination) because of its
small core size and small numerical aperture. Because of the high coupling losses associated with
LEDs, single-mode fiber is used primarily with laser diodes as a source.
Graded-index fiber
In a step-index fiber, the refractive index of the core has a constant value. By contrast, in a
graded-index fiber, the refractive index in the core decreases continuously (in a parabolic
fashion) from a maximum value at the center of the core to a constant value at the core-cladding
interface. (See Figure 4-13c.) Graded-index fiber is characterized by its ease of use (i.e., large
core diameter and N.A.), similar to a step-index multimode fiber, and its greater information
carrying capacity, as in a step-index single-mode fiber. Light traveling through the center of the
fiber experiences a higher index of refraction than does light traveling in higher modes. This
means that even though the higher-order modes must travel farther than the lower order modes,
they travel faster, thus decreasing the amount of modal dispersion and increasing the bandwidth
of the fiber.
Polarization-maintaining fiber
Polarization-maintaining (PM) fiber is a
type of fiber that only allows light of a
specific polarization orientation to
propagate. It is often referred to as high
birefringence single-mode fiber. (A
birefringent material is one in which the
refractive index is different for two
orthogonal orientations of the light
propagating through it.) In birefringent
fiber, light polarized in orthogonal
directions will travel at different speeds
along the polarization axes of the fiber. PM
fibers utilize a stress-induced birefringence
mechanism to achieve high levels of
birefringence. These fibers embed a stressapplying region in the cladding area of the
fiber. (See Figure 4-14.) Placed
symmetrically about the core, it gives the
Figure 4-14 Polarization maintaining fiber
Module 2-4: Principles of Fiber Optic Communication
23
fiber cross-section two distinct axes of symmetry. The stress region squeezes on the core along
one axis, which makes the core birefringent. As a result, the propagation speed is polarizationdependent, differing for light polarized along the two orthogonal symmetry axes. Birefringence
is the key to polarization-maintaining behavior. Because of the difference in propagation speed,
light polarized along one symmetry axis is not efficiently coupled to the other orthogonal
polarization—even when the fiber is coiled, twisted or bent. PM fibers can be designed with
high stress levels to create birefringence sufficient to resist depolarization under harsh
mechanical and thermal operating conditions.
Fiber Optic Cable Design
In most applications, optical fiber is protected from the environment by using a variety of
different cabling types based on the type of environment in which fiber will be used. Cabling
provides the fiber with protection from the elements, added tensile strength for pulling, rigidity
for bending, and durability. As fiber is drawn from the preform in the manufacturing process, a
protective coating, a UV-curable acrylate, is applied to protect against moisture and to provide
mechanical protection during the initial stages of cabling. A secondary buffer then typically
encases the optical fibers for further protection.
Fiber optic cable can be separated into two types: indoor and outdoor cables. (See Table 4-1.)
Table 4-1. Indoor and Outdoor Cables
Indoor Cable
Simplex Cables
Duplex Cables
Multifiber Cables
Breakout Cables
Heavy, Light,
Plenum-Duty, and
Riser Cable
Outdoor Cables
Overhead
Direct Burial
Indirect Burial
Submarine
Description
Contains a single fiber for one-way communication
Contains two fibers for two-way communications
Contains more than two fibers. Fibers are usually in pairs for duplex operation.
For example, a twelve-fiber cable permits six duplex circuits.
Typically have several individual simplex cables inside an outer jacket. The
outer jacket includes a ripcord to allow easy access
Heavy-duty cables have thicker jackets than light duty cable for rougher
handling
Plenum cables are jacketed with low-smoke and fire retardant materials
Riser cables run vertically between floors and must be engineered to prevent
fires from spreading between floors
Outdoor cables must withstand more harsh environment conditions than
indoor cables.
Cables strung from telephone lines
Cables placed directly in a trench
Cables placed in a conduit
Underwater cable, including transoceanic applications
Most telecommunication applications employ either a loose-tube, tight buffer, or ribbon-cable
design. Loose tube cable is used primarily in outside-plant applications that require high pulling
strength, resistance to moisture, large temperature ranges, low attenuation, and protection from
other environmental factors. (See Figure 4-15.) Loose-tube buffer designs allow easy drop-off
of groups of fibers at intermediate points. A typical loose-tube cable can hold up to 12 fibers,
with a cable capacity of more than 200 fibers. In a loose-tube cable design, color-coded plastic
24
Optics and Photonics Series, Course 2: Elements of Photonics
buffer tubes are filled with a gel to provide protection from water and moisture. The fact that the
fibers “float” inside the tube provides additional isolation from mechanical stress such as pull
force and bending introduced during the installation process. Loose-tube cables can be either all
dielectric, or armored. In addition, the buffer tubes are stranded around a dielectric or steel
central member which serves as an anti-buckling element. The cable core is typically
surrounded by aramid fibers to provide tensile strength to the cable. For additional protection, a
medium-density outer polyethylene jacket is extruded over the core. In armored designs,
corrugated steel tape is formed around a single-jacketed cable with an additional jacket extruded
over the armor.
Figure 4-15 Loose tube direct burial cable
Tight buffer cable is typically used for indoor applications where ease of cable termination and
flexibility are more of a concern than low attenuation and environmental stress. (See
Figure 4-16.) In a tight-buffer cable, each fiber is individually buffered (direct contact) with an
elastomeric material to provide good impact resistance and flexibility, while keeping size at a
minimum. Aramid fiber strength members provide the tensile strength for the cable. This type of
cable is suited for “jumper cables”, which typically connect loose-tube cables to active
components such as lasers and receivers. Tight-buffer fiber may introduce slightly more
attenuation due to the stress placed on the fiber by the buffer. However, because tight-buffer
cable is typically used for indoor applications, distances are generally much shorter that for
outdoor applications allowing systems to tolerate more attenuation in exchange for other
benefits.
Module 2-4: Principles of Fiber Optic Communication
25
Figure 4-16 Tight buffer simplex and duplex cable
Ribbon cable is used in applications where fibers must be densely packed. (See Figure 4-17.)
Ribbon cables typically consist of up to 18-coated fibers that are bonded or laminated to form a
ribbon. Many ribbons can then be combined to form a thick, densely packed fiber cable that can
be either mass-fusion spliced or terminated using array connectors that can save a considerable
amount of time as compared to loose-tube or tight-buffer designs.
Figure 4-17 Loose tube ribbon cable
26
Optics and Photonics Series, Course 2: Elements of Photonics
Cabling example
Figure 4-18 shows an example of an interbuilding cabling scenario.
Figure 4-18 Interbuilding cabling scenario
Dispersion
In digital communication systems, information to be sent is first coded in the form of pulses.
These pulses of light are then transmitted from the transmitter to the receiver, where the
information is decoded. The larger the number of pulses that can be sent per unit time and still
be resolvable at the receiver end, the larger will be the transmission capacity, or bandwidth of
the system. A pulse of light sent into a fiber broadens in time as it propagates through the fiber.
This phenomenon is known as dispersion, and is illustrated in Figure 4-19.
Module 2-4: Principles of Fiber Optic Communication
27
Figure 4-19 Pulses separated by 100 ns at the input end would be resolvable at the output end of 1 km
of the fiber. The same pulses would not be resolvable at the output end of 2 km of the same fiber.
Calculating dispersion
Dispersion, termed ∆t, is defined as pulse spreading in an optical fiber. As a pulse of light
propagates through a fiber, elements such as numerical aperture, core diameter, refractive index
profile, wavelength, and laser linewidth cause the pulse to broaden. This poses a limitation on
the overall bandwidth of the fiber as demonstrated in Figure 4-20.
Figure 4-20 Pulse broadening caused by dispersion
Dispersion ∆t can be determined from Equation 4-20.
∆t = (∆tout – ∆tin)1/2
(4-20)
Dispersion is measured in units of time, typically nanoseconds or picoseconds. Total dispersion
is a function of fiber length, ergo, the longer the fiber, the more the dispersion. Equation 4-21
gives the total dispersion per unit length.
∆ttotal = L × (Dispersion/km)
(4-21)
The overall effect of dispersion on the performance of a fiber optic system is known as
intersymbol interference, as shown in Figure 4-19. Intersymbol interference occurs when the
pulse spreading due to dispersion causes the output pulses of a system to overlap, rendering
them undetectable. If an input pulse is caused to spread such that the rate of change of the input
exceeds the dispersion limit of the fiber, the output data will become indiscernible.
28
Optics and Photonics Series, Course 2: Elements of Photonics
Intermodal dispersion
Intermodal dispersion is the pulse spreading caused by the time delay between lower-order
modes (modes or rays propagating straight through the fiber close to the optical axis) and
higher-order modes (modes propagating at steeper angles). This is shown in Figure 4-21. Modal
dispersion is problematic in multimode fiber and is the primary cause for bandwidth limitation.
It is not a problem in single-mode fiber where only one mode is allowed to propagate.
Figure 4-21 Mode propagation in an optical fiber
Chromatic dispersion
Chromatic dispersion is pulse spreading due to the fact that different wavelengths of light
propagate at slightly different speeds through the fiber. All light sources, whether laser or LED,
have finite linewidths, which means they emit more than one wavelength. Because the index of
refraction of glass fiber is a wavelength-dependent quantity, different wavelengths propagate at
different speeds. Chromatic dispersion is typically expressed in units of nanoseconds or
picoseconds per (km-nm).
Chromatic dispersion consists of two parts: material dispersion and waveguide dispersion.
∆tchromatic = ∆tmaterial + ∆twaveguide
(4-22)
Material dispersion is due to the wavelength dependency on the index of refraction of glass.
Waveguide dispersion is due to the physical structure of the waveguide. In a simple step-indexprofile fiber, waveguide dispersion is not a major factor, but in fibers with more complex index
profiles, waveguide dispersion can be more significant. Material dispersion and waveguide
dispersion can have opposite signs (or slopes) depending on the transmission wavelength. In the
case of a step-index single-mode fiber, these two effectively cancel each other at 1310 nm
yielding zero-dispersion, which makes high-bandwidth communication possible at this
wavelength. The drawback, however, is that even though dispersion is minimized at 1310 nm,
attenuation is not. Glass fiber exhibits minimum attenuation at 1550 nm. Glass exhibits its
minimum attenuation at 1550 nm, and optical amplifiers (known as erbium-doped fiber
amplifiers [EDFA]) also operate in the 1550-nm range. It makes sense, then, that if the zerodispersion property of 1310 nm could be shifted to coincide with the 1550-nm transmission
window, very high-bandwidth long-distance communication would be possible. With this in
mind, zero-dispersion-shifted fiber was developed.
Zero-dispersion-shifted fiber “shifts “the zero dispersion wavelength of 1310 nm to coincide
with the 1550 nm transmission window of glass fiber by modifying the waveguide dispersion
slope. Modifying the waveguide dispersion slope is accomplished by modifying the refractive
Module 2-4: Principles of Fiber Optic Communication
29
index profile of the fiber in a way that yields a more negative waveguide-dispersion slope.
When combined with a positive material dispersion slope, the point at which the sum of two
slopes cancel each other out can be shifted to a higher wavelength such as 1550 nm or beyond.
(See Figure 4-22.)
Figure 4-22 Single-mode versus dispersion-shifted fiber
An example of a zero-dispersion-shifted fiber is the “W-profile” fiber, named because of the
shape of the refractive index profile which looks like a “W.” This is illustrated in Figure 4-23.
By splicing in short segments of a dispersion-shifted fiber with the appropriate negative slope
into a fiber optic system with positive chromatic dispersion, the pulse spreading can be
minimized. This results in an increase in data rate capacity.
Figure 4-23 W-profile fibers: (a) step-index, (b) triangular profile
In systems where multiple wavelengths are transmitted through the same single-mode fiber,
such as in dense wavelength division multiplexing (DWDM, discussed in a later section), it is
possible for three equally spaced signals transmitted near the specified zero-dispersion
wavelength to combine and generate a new fourth wave, which can cause interference between
channels. This phenomenon is called four-wave mixing, which degrades system performance. If,
however, the waveguide structure of the fiber is modified so that the waveguide dispersion is
further increased in the negative direction, the zero-dispersion point can be pushed out past
1600 nm (outside the EDFA operating window). This results in a fiber in which total chromatic
dispersion is still substantially lower in the 1550 nm range without the threat of performance
problems. This type of fiber is known as nonzero dispersion-shifted fiber.
30
Optics and Photonics Series, Course 2: Elements of Photonics
The total dispersion of an optical fiber, ∆ttot, can be approximated using
∆ttotal = ∆t 2 modal + ∆t 2 chromatic
(4-23)
where ∆tmodal represents the dispersion due to the various components that make up the system.
The transmission capacity of fiber is typically expressed in terms of bandwidth × distance. For
example, the (bandwidth × distance) product for a typical 62.5/125-µm (core/cladding diameter)
multimode fiber operating at 1310 nm might be expressed as 600 MHz • km.
The approximate bandwidth BW of a fiber can be related to the total dispersion by the following
relationship:
BW (Hz) = 0.35/∆ttotal
(4-24)
Example 9
A 2-km-length multimode fiber has a modal dispersion of 1 ns/km and a chromatic dispersion of
100 ps/km • nm. It is used with an LED of linewidth 40 nm. (a) What is the total dispersion?
(b) Calculate the bandwidth (BW) of the fiber.
(a)
∆tmodal = 2 km × 1 ns/km = 2 ns
∆tchromatic = (2 km) × (100 ps/km • nm) × (40 nm) = 8000 ps = 8 ns
Now, from Equation 4-23,
∆ttotal = ([2 ns]2 + [8 ns]2 )1/2 = 8.25 ns
And from Equation 4-24,
(b) BW = 0.35/∆ttotal = 0.35/8.25 ns = 42.42 MHz
Expressed in terms of the product (BW • km), we get (BW • km) = (42.4 MHz)( 2 km) ≅ 85 MHz • km.
Example 10
A 50-km single-mode fiber has a material dispersion of 10 ps/km • nm and a waveguide dispersion
of –5 ps/km • nm. It is used with a laser source of linewidth 0.1 nm. (a) What is ∆tchromatic? (b) What
is ∆ttotal? (c) Calculate the bandwidth (BW) of the fiber.
(a)
With the help of Equation 4-22, we get
∆tchromatic = 10 ps/km • nm – 5 ps/km • nm = 5 ps/km • nm
(b) For 50 km of fiber at a linewidth of 0.1 nm, ∆ttotal is
∆ttotal = (50 km) × (5 ps/km • nm) × (0.1 nm) = 25 ps
(b) BW = 0.35/∆ttotal = 0.35/25 ps = 14 GHz
Expressed in terms of the product (BW • km), we get
(BW • km) = (14 GHz)(50 km) = 700 GHz • km
In short, the fiber in this example could be operated at a data rate as high as 700 GHz over a onekilometer distance.
Module 2-4: Principles of Fiber Optic Communication
31
Fiber Optic Sources
Two types of light sources are commonly used in fiber optic communications systems:
semiconductor laser diodes (LD) and light-emitting diodes (LED). Each device has its own
advantages and disadvantages as listed in Table 4-2.
Table 4-2. LED Versus Laser
Characteristic
Output power
Spectral width
Numerical aperture
Speed
Cost
Ease of operation
LED
Laser (LD)
Lower
Wider
Larger
Slower
Less
Easier
Higher
Narrower
Smaller
Faster
More
More difficult
Fiber optic sources must operate in the low-loss transmission windows of glass fiber. LEDs are
typically used at the 850-nm and 1310-nm transmission wavelengths, whereas lasers are
primarily used at 1310 nm and 1550 nm.
LEDs
LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their
inherent bandwidth limitations and lower output power. They are used in applications in which
data rates are in the hundreds of megahertz as opposed to GHz data rates associated with lasers.
Two basic structures for LEDs are used in fiber optic systems: surface-emitting and edgeemitting as shown in Figure 4-24.
Figure 4-24 Surface-emitting versus edge-emitting diodes
LEDs typically have large numerical apertures, which makes light coupling into single-mode
fiber difficult due to the fiber’s small N.A. and core diameter. For this reason LEDs are most
often used with multimode optical fiber. LEDs are used in lower-data-rate, short-distance
(<1 km) multimode systems because of their inherent bandwidth limitations and low output
power. In addition, the output spectrum of a typical LED is about 40 nm, which limits its
performance due to severe chromatic dispersion. LEDs, however, operate in a more linear
fashion than do laser diodes making them more suitable for analog modulation. Most fiber optic
light sources are pigtailed, having a fiber attached during the manufacturing process. Some
32
Optics and Photonics Series, Course 2: Elements of Photonics
LEDs are available with connector-ready housings that allow a connectorized fiber to be
directly attached and are relatively inexpensive compared to laser diodes. LEDs are used in
applications including local area networks, closed-circuit TV, and where transmitting electronic
data in areas where EMI may be a problem.
Laser diodes
Laser diodes are used in applications in which longer distances and higher data rates are
required. Because an LD has a much higher output power than an LED, it is capable of
transmitting information over longer distances. Consequently, and given the fact that the LD has
a much narrower spectral width, it can provide high-bandwidth communication over long
distances. The LD’s smaller N.A. also allows it to be more effectively coupled with single-mode
fiber. The difficulty with LDs is that they are inherently nonlinear, which makes analog
transmission more difficult. They are also very sensitive to fluctuations in temperature and drive
current, which causes their output wavelength to drift. In applications such as wavelengthdivision multiplexing in which several wavelengths are being transmitted down the same fiber,
the wavelength stability of the source becomes critical. This usually requires complex circuitry
and feedback mechanisms to detect and correct for drifts in wavelength. The benefits, however,
of high-speed transmission using LDs typically outweigh the drawbacks and added expense.
In high-speed telecommunications applications, specially designed single-frequency diode lasers
that operate with an extremely narrow output spectrum (< .01 nm) are required. These are
known as distributed-feedback (DFB) laser diodes (Figure 4-25). In DFB lasers, a corrugated
structure, or diffraction grating, is fabricated directly in the cavity of the laser, allowing only
light of a very specific wavelength to oscillate. This yields an output wavelength spectrum that
is extremely narrow—a characteristic required for dense wavelength division-multiplexing
(DWDM) systems in which many closely spaced wavelengths are transmitted through the same
fiber. Distributed-feedback lasers are available at fiber optic communication wavelengths
between 1300 nm and 1550 nm.
Figure 4-25 Fourteen-pin butterfly mount distributed feedback laser diode (Source: JDS Uniphase
Corporation; used by permission)
Module 2-4: Principles of Fiber Optic Communication
33
Fiber Optic Detectors
The purpose of a fiber optic detector is to convert light emanating from the optical fiber back
into an electrical signal. The choice of a fiber optic detector depends on several factors
including wavelength, responsivity, and speed or rise time. Figure 4-26 depicts the various types
of detectors and their spectral responses.
Figure 4-26 Detector spectral response
The process by which light energy is converted into an electrical signal is the opposite of the
process by which an electrical signal is converted into light energy. Light striking the detector
generates a small electrical current that is amplified by an external circuit. Photons absorbed in
the PN junction of the detector excite electrons from the valence band to the conduction band,
resulting in the creation of an electron-hole pair. Under the influence of a bias voltage these
carriers move through the material and induce a current in the external circuit. For each
electron-hole pair created, the result is an electron flowing in the circuit. Current levels are
usually small and require some amplification as shown in Figure 4-27.
Figure 4-27 Typical detector amplifier circuit
34
Optics and Photonics Series, Course 2: Elements of Photonics
The most commonly used photodetectors in fiber optic communication systems are the PIN and
avalanche photodiodes (APD). The material composition of the device determines the
wavelength sensitivity. In general, silicon devices are used for detection in the visible portion of
the spectrum. InGaAs crystals are used in the near-infrared portion of the spectrum between
1000 nm and 1700 nm. Germanium PIN and APDs are used between 800 nm and 1500 nm.
Table 4-3 gives some typical photodetector characteristics:
Table 4-3. Typical Photodetector Characteristics
Photodetector
Silicon PIN
InGaAs PIN
InGaAs APD
Wavelength (nm)
250–1100
1310–1625
1310–1625
Responsivity (A/W)
0.1–1.0
0.3–0.85
0.7–1.0
Dark Current (nA)
1–10
0.1–1
30–200
Rise Time (ns)
0.07
0.03
0.03
Some of the more important detector parameters listed below in Table 4-4 are defined and
described in Module 1-6, Optical Detectors and Human Vision.
Table 4-4. Photodetector Parameters
Parameter
Description
Responsivity
The ratio of the electrical power to the detector’s output optical power
Quantum
efficiency
The ratio of the number of electrons generated by the detector to the
number of photons incident on the detector
Quantum efficiency = (Number of electrons)/Photon
Dark current
The amount of current generated by the detector with no light applied.
Dark current increases about 10% for each temperature increase of 1°C
and is much more prominent in Ge and InGaAs at longer wavelengths
than in silicon at shorter wavelengths.
Noise floor
The minimum detectable power that a detector can handle. The noise
floor is related to the dark current since the dark current will set the lower
limit.
Noise floor = Noise (A)/Responsivity (A/W)
Response Time
The time required for the detector to respond to an optical input. The
response time is related to the bandwidth of the detector by
BW = 0.35/tr
where tr is the rise time of the device. The rise time is the time required
for the detector to rise to a value equal to 63.2% of its final steady-state
reading.
Noise equivalent
power (NEP)
At a given modulation frequency, wavelength, and noise bandwidth, NEP
is the incident radiant power that produces a signal-to-noise ratio of one
at the output of the detector
Module 2-4: Principles of Fiber Optic Communication
35
Connectors
In the 1980s, there were many different types and manufacturers of connectors. Some remain in
production, but much of the industry has shifted to standardized connector types, with details
specified by standards organizations such as the Telecommunications Industry Association, the
International Electro-technical Commission, and the Electronic Industry Association. Today,
there are many different types of connectors available for fiber optics depending on the
application. Some of the more common types are shown in Table 4-5:
Table 4-5. Fiber Optic Connector Types
(Source of photos: JDS Uniphase Corporation; used by permission)
Type
36
Description
SC
Snap-in Single-Fiber Connector: A square
cross section allows high packing density on
patch panels and makes it easy to package in
a polarized duplex form that assures the fibers
are matched to the proper fibers in the mated
connector. Used in premise cabling, ATM,
fiber-channel, and low-cost FDDI. Available in
simplex and duplex configurations.
ST
Twist-on Single-Fiber Connector: The most
widely used and broadly used type of
connector for data communications
applications. A bayonet-style “twist and lock”
coupling mechanism allows for quick connects
and disconnects, and a spring-loaded 2.5 mm
diameter ferrule for constant contact between
mating fibers
LC
Small Form Factor Connector: Similar to SC
connector but designed to reduce system
costs and connector density.
FC
Twist-on Single-Fiber Connector: Similar to the
ST connector and used primarily in the
telecommunications industry. A threaded
coupling and tunable keying allows ferrule to
be rotated to minimize coupling loss.
Diagram
Optics and Photonics Series, Course 2: Elements of Photonics
Regardless of the type of connector used, compatibility between connectors produced by different
manufacturers is essential. This does not necessarily mean that the connectors are identical.
Connectors produced by different manufacturers may differ in the number of parts, ease and
method of terminations, material used, and whether epoxy is used. Connectors may also differ in
their performance involving insertion loss, durability, return loss, temperature range, etc.
Single-mode and multimode connectors may also vary in terms of ferrule bore tolerance. A
standard 125-um single-mode requires a more exacting fit to minimize insertions loss, whereas a
multimode fiber with its larger core may be more forgiving. A typical multimode connector may
have a bore diameter as large as 127 um to accommodate the largest fiber size. A single-mode
connector, however, may be specified with a bore diameter of 125, 126, or 127 um to ensure a
more precise fit.
Fiber Optic Couplers
A fiber optic coupler is a device used to connect a single (or multiple) fiber to many other
separate fibers. There are two general categories of couplers:
•
Star couplers (Figure 4-28a)
•
T-couplers (Figure 4-28b)
Figure 4-28 (a) Star coupler (b) T-coupler
Star couplers
In a star coupler, each of the optical signals sent into the coupler are available at all of the
output fibers (Figure 4-28a). Power is distributed evenly. For an n × n star coupler (n-inputs and
n-outputs), the power available at each output fiber is 1/n the power of any input fiber.
The output power from a star coupler is simply
Po = Pin/n
(4-25)
where n = number of output fibers.
The power division (or power splitting ratio) PDst in decibels is given by Equation 4-26.
PDst(dB) = –10 log(1/n)
Module 2-4: Principles of Fiber Optic Communication
(4-26)
37
The power division in decibels gives the number of decibels apparently lost in the coupler from
single input fiber to single fiber output. Excess power loss (Lossex) is the power lost from input
to total output, as given in Equation 4-27 or 4-28.
Loss ex =
Pout (total)
Pin
Loss ex/dB = –10 log
Pout (total)
Pin
(4-27)
(4-28)
Example 11
An 8 × 8 star coupler is used in a fiber optic system to connect the signal from one computer to
eight terminals. The power at an input fiber to the star coupler is 0.5 mW. Find (1) the power at each
output fiber and (2) the power division in decibels.
Solution
(1) The 0.5-mW input is distributed to eight fibers. Each has (0.50 mW)/8 = 0.0625 mW.
(2) The power division, in decibels, from Equation 4-26 is
PDst = –10 × log(1/8) = 9.0 dB
Example 12
A 10 × 10 star coupler is used to distribute the 3-dBm power of a laser diode to 10 fibers. The
excess loss (Lossex) of the coupler is 2 dB. Find the power at each output fiber in dBm and µW.
Solution
The power division in dB from Equation 4-26 is
PDst = –10 × log (1/10) = 10 dB
To find Pout for each fiber, subtract PDst and Lossex from Pin in dBm:
∴ Pout = 3 dBm – 10 dB – 2 dB = –9 dBm
To find Pout in watts we use Equation 4-13:
⎛ P ⎞
–9 = 10 × log ⎜ out ⎟
⎝ 1 mW ⎠
−9
Pout
= 10 10
1 mW
∴ Pout = (1 mW)(10–0.9)
Pout = (10–3)(0.126) W
Solving, we get
Pout = 126 µW
38
Optics and Photonics Series, Course 2: Elements of Photonics
An important characteristic of star couplers is cross talk or the amount of input information
coupled into another input. Cross coupling is given in decibels (typically greater than 40 dB).
T-couplers
Figure 4-29 show a T-coupler. Power is launched into port 1 and is then split between ports 2
and 3. The power split does not have to be equal. The power division is given in decibels or in
percent. For example, an 80/20 split means 80% to port 2, 20% to port 3. In decibels, this
corresponds to 0.97 dB for port 2 and almost 7.0 dB for port 3.
Figure 4-29 T-coupler
10 log (P2/P1) = –0.97 dB
10 log (P3/P1) = –6.99 dB
Directivity describes the transmission between the ports. For example, if P3/P1 = 0.5, P3/P2 does
not necessarily equal 0.5. For a highly directive T-coupler, P3/P2 is very small. That is, no
power is transferred between the two ports on the same side of the coupler.
Wavelength-division multiplexers
The couplers used for wavelength-division multiplexing (WDM) are designed specifically to
make the coupling between ports a function of wavelength. The purpose of these couplers is to
separate (or combine) signals transmitted at different wavelengths. Essentially, the transmitting
coupler is a mixer and the receiving coupler is a wavelength filter. Wavelength-division
multiplexers use several methods to separate different wavelengths depending on the spacing
between the wavelengths. Separation of
1310 nm and 1550 nm is a simple operation
and can be achieved with WDMs that employ
bulk optical diffraction gratings. Wavelengths
in the 1550-nm range that are spaced at greater
than 1 to 2 nm can be resolved using WDMs
that incorporate interference filters. To
separate very closely spaced wavelengths
(< 0.8 nm) in a dense wavelength-division
multiplexing system (DWDM), however, fiber
Bragg gratings are required. An example of an
8-channel WDM is shown in Figure 4-30.
Figure 4-30 Eight-channel WDM (Source: DiCon
Fiberoptics, Inc.; used by permission)
DWDM refers to the transmission of multiple
closely spaced wavelengths through the same
fiber. (See Figure 4-31.) For any given wavelength λ and corresponding frequency f, the
International Telecommunications Union (ITU) defines standard frequency spacing ∆f as
Module 2-4: Principles of Fiber Optic Communication
39
100 GHz, which translates into a ∆λ of 0.8-nm wavelength spacing. This follows from the
λ ∆f
. (See Table 4-6.) DWDM systems operate in the 1550-nm window
relationship ∆λ =
f
because of the low attenuation characteristics of glass at 1550 nm and the fact that erbiumdoped fiber amplifiers (EDFA) operate in the 1530-nm to 1570-nm range. Although the ITU
grid specifies that each transmitted wavelength in a DWDM system is separated by 100 GHz,
systems are currently available with channel spacing to 50 GHz and below (< 0.4 nm). As the
channel spacing decreases, the number of channels that can be transmitted increases, thus
further increasing the transmission capacity of the system.
Figure 4-31 Wavelength-division multiplexing
Table 4-6. ITU grid
Center Wavelength – nm
(vacuum)
Optical Frequency
(THz)
1546.92
1547.72
193.8
193.7
1530.33
1531.12
1531.90
1532.68
1533.47
1534.25
1535.04
1535.82
1536.61
1537.40
1538.19
1538.98
1539.77
1540.56
1541.35
1542.14
1542.94
1543.73
1544.53
1545.32
1546.12
195.9
195.8
195.7
195.6
195.5
195.4
195.3
195.2
195.1
195.0
194.9
194.8
194.7
194.6
194.5
194.4
194.3
194.2
194.1
194.0
193.9
1548.51
1549.32
1550.12
1550.92
1551.72
1552.52
1553.33
1554.13
1554.93
1555.75
1556.55
1557.36
1588.17
1558.98
1559.79
1560.61
1561.42
1562.23
1563.05
1563.86
193.6
193.5
193.4
193.3
193.2
193.1
193.0
192.9
192.8
192.7
192.6
192.5
192.4
192.3
192.2
192.1
192.0
191.9
191.8
191.7
40
Optics and Photonics Series, Course 2: Elements of Photonics
Fiber Bragg gratings
Fiber Bragg gratings are devices that are used in DWDM systems in which multiple closely
spaced wavelengths require separation. (See Figure 4-32.) Light entering the fiber Bragg grating
is reflected by periodic variations in the index of refraction in the fiber’s core. Fiber Bragg
gratings are fabricated by passing ultraviolet light from an excimer laser through either a phase
mask or diffraction and exposing a short segment of optical fiber with its core doped with a
photosensitive material. Periodic variations in intensity incident on the fiber caused by the mask
or diffraction grating create periodic variations in refractive index in the fiber core. By choosing
the appropriate spacing between the periodic variations to be multiples of the half-wavelength
of the desired signal, each variation reflects light with a 360° phase shift causing a constructive
interference of a very specific wavelength while allowing others to pass.
Figure 4-32 Fiber Bragg grating
Fiber Bragg gratings are available with bandwidths ranging from 0.01 nm up to 20 nm. They are
typically used in conjunction with circulators, which are used to extract or “drop” single or
multiple narrow-band WDM channels while transmitting other channels (see Figure 4-33). Fiber
Bragg gratings have emerged as a major factor, along with EDFAs, in increasing the capacity of
next-generation high-bandwidth fiber optic systems.
Figure 4-33 Fiber optic circulator
Module 2-4: Principles of Fiber Optic Communication
41
Erbium-doped fiber amplifiers (EDFA)
The EDFA is an optical amplifier used to boost the signal level in the 1530-nm to 1570-nm
region of the spectrum. When it is pumped by an external laser source of either 980 nm or
1480 nm, signal gain can be as high as 30 dB (1000 times). Because EDFAs allow signals to be
regenerated without having to be converted back to electrical signals, systems are faster and
more reliable. When used in conjunction with wavelength-division multiplexing, fiber optic
systems can transmit enormous amounts of information over long distances with very high
reliability. (See Figure 4-34.)
Figure 4-34 Wavelength-division multiplexing system using EDFAs
Fiber Optic Sensors
Although the most important application of optical fibers is in the field of transmission of
information, optical fibers capable of sensing various physical parameters and generating
information are also finding widespread use. The use of optical fibers for such applications
offers the same advantages as in the field of communication: lower cost, smaller size, more
accuracy, greater flexibility, and greater reliability. As compared to conventional electrical
sensors, fiber optic sensors are immune to external electromagnetic interference and can be used
in hazardous and explosive environments. A very important attribute of fiber optic sensors is the
possibility of having distributed or quasi-distributed sensing geometries, which would otherwise
be too expensive or complicated using conventional sensors. With fiber optic sensors it is
possible to measure pressure, temperature, electric current, rotation, strain, and chemical and
biological parameters with greater precision and speed. These advantages are leading to
increased integration of such sensors in civil engineering structures such as bridges and tunnels,
in process industries, medical instruments, aircraft, missiles, and even cars.
Fiber optic sensors can be broadly classified into two categories: extrinsic and intrinsic. In the
case of extrinsic sensors, the optical fiber simply acts as a device to transmit and collect light
from a sensing element, which is external to the fiber. The sensing element responds to the
external perturbation, and the change in the characteristics of the sensing element is transmitted
by the return fiber for analysis. The optical fiber here plays no role other than that of
transmitting the light beam. On the other hand, in the case of intrinsic sensors, the physical
parameter to be sensed directly alters the properties of the optical fiber, which in turn leads to
42
Optics and Photonics Series, Course 2: Elements of Photonics
changes in a characteristic such as intensity, polarization, or phase of the light beam propagating
in the fiber.
A large variety of fiber optic sensors have been demonstrated in the laboratory, and many are
already being installed in real systems. In the following sections, we will discuss some
important examples of fiber optic sensors.
Extrinsic fiber optic sensors
Figure 4-35 shows a very simple sensor based on the fact that transmission through a fiber joint
depends on the alignment of the fiber cores. Light coupled into a multimode optical fiber
couples across a joint into another fiber. The light is detected by a photodetector. The detector
immediately senses any deviation of the fiber pair from perfect alignment. A misalignment of
magnitude equal to the core diameter of the fiber results in zero transmission. The first 20% of
transverse displacement gives an approximately linear output. Thus, for a 50-µm-core-diameter
fiber, approximately 10-µm misalignment will be linear. The sensitivity will of course become
better with decrease in core diameter, but, at the same time, the range of displacements will also
reduce.
Figure 4-35 A change in the transverse alignment between two fibers changes the coupling and hence
the power falling on the detector
The misalignment between the fibers could be caused by various physical parameters, such as
acoustic waves and pressure. Thus, if one of the probe fibers has a short free length while the
other has a longer length, acoustic waves impinging on the sensor will set the fibers into
vibration, which will result in a modulation of the transmitted light intensity leading to an
acoustic sensor. Using such an arrangement, deep-sea noise levels in the frequency range of
100 Hz to 1 kHz and transverse displacements of a few tenths of a nanometer have been
measured. Using the same principle, any physical parameter leading to a relative displacement
of the fiber cores can be sensed using this geometry.
Figure 4-36 shows a modification of the sensor in the form of a probe. Here light from an LED
coupled into a multimode fiber passes through a fiber optic splitter to the probe. The probe is in
the form of a reflecting diaphragm in front of the fiber, as shown. Light emanating from the
fiber is reflected by the diaphragm, passes again through the splitter, and is detected by a
photodetector. Any change in the external pressure causes the diaphragm to bend, leading to a
change in the power coupled into the fiber. Such sensors can be built to measure pressure
variations in medical as well as other applications requiring monitoring operating pressures of
up to 4 mega Pascal (~ 600 psi). Such a device can be used for the measurement of pressure in
the arteries, bladder, urethra, etc.
Module 2-4: Principles of Fiber Optic Communication
43
Figure 4-36 Light returning to the detector changes as the shape of the reflecting diaphragm changes
due to changes in external pressure.
If the diaphragm at the output is removed and the light beam is allowed to fall on the sample,
light that is reflected or scattered is again picked up by the fiber and detected and processed by
the detector. With analysis of the returning optical beam, information about the physical and
chemical properties of the blood can be obtained. Thus, if the scattering takes place from
flowing blood, the scattered light beam is shifted in frequency due to the Doppler effect.
(Doppler effect refers to the apparent frequency shift of a wave detected by an observer—
compared with its true frequency—when there is relative motion between source and observer.
You may have noticed the falling frequency of the whistle of a train as it approaches and passes
by you.). The faster the blood cells are moving, the larger will be the shift. Through
measurement of the shift in frequency, the blood flow rate can be estimated. By a spectroscopic
analysis of the returning optical signal, one can estimate the oxygen content in the blood. One of
the most important advantages of using optical fibers in this process is that they do not provoke
adverse response from the immune system. They are more durable, more flexible, and
potentially safer than alternatives.
Another very interesting sensor is the liquid-level sensor shown in Figure 4-37. Light
propagating down an optical fiber is total internally reflected from a small glass prism and
couples back to the return fiber. As long as the external medium is air, the angle of incidence
inside the prism is greater than the critical angle and hence light suffers total internal reflection.
As soon as the prism comes in contact with a liquid, the critical angle at the prism-liquid
interface reduces and the light is transmitted into the liquid, resulting in a loss of signal. By a
proper choice of prism material, such a sensor can be used for sensing levels of various liquids
such as water, gasoline, acids, and oils.
Figure 4-37 A liquid-level sensor based on changes in the critical angle due to liquid level moving up
to contact the sides of the prism
44
Optics and Photonics Series, Course 2: Elements of Photonics
Example 13
For a prism with refractive index np of 1.5, the critical angles with air (na = 1.0) and water (nw = 1.33) are
41.8o and 62.7o respectively. Thus, if the prism is isosceles right-angled, with two angles as 45o, light
that suffers total internal reflection with air as the surrounding medium will suffer only partial internal
reflection with water as the surrounding medium, resulting in a loss of signal.
Intrinsic sensors
In intrinsic sensors the physical parameter changes some characteristic of the propagating light
beam that is sensed. Among the many intrinsic sensors, here we discuss two important
examples, namely the Mach-Zehnder interferometric fiber sensor and the fiber optic gyroscope.
Mach-Zehnder interferometric sensor—One of the most sensitive arrangements for a
fiber optic sensor is the Mach-Zehnder (MZ) interferometric sensor arrangement shown in
Figure 4-38. Light from a laser is passed through a 3-dB fiber optic coupler, which splits the
incoming light beam into two equal-amplitude beams in the two single-mode fiber arms. The
light beams recombine at the output coupler after passing through the two arms. The output
from the output coupler is detected and processed. One of the fiber arms of the interferometer is
the sensing arm, which is sensitive to the external parameter to be sensed. The other fiber arm is
the reference arm. It is usually coated with a material to make it insensitive to the parameter of
measurement. The two fiber arms behave as two paths of an interferometer, and hence the
output depends on the phase difference between the beams as they enter the output coupler. If
the two fibers are of exactly equal lengths, the entire input light beam appears in the lower fiber
and no light comes out of the upper fiber. Any external parameter such as temperature or
pressure affects the sensing fiber by changing either the refractive index or the length of the
arm, thus changing the phase difference between the two beams as they enter the output coupler.
This results in a change in the intensity of the two output arms. Processing of the output leads to
a measurement of the external parameter.
Figure 4-38 Fiber optic Mach-Zehnder interferometric sensor. Phase changes (due to external
perturbation on the sensing arm) between the light beams arriving at the output coupler cause changes
in intensity at the output.
Module 2-4: Principles of Fiber Optic Communication
45
The MZ sensor is extremely sensitive to external perturbations. For example, the change of
phase due to an external pressure that causes both a change in refractive index and a change in
the length of the specially coated sensing arm is about 3 × 10–4 rad/Pa-m. Here Pa = 1 N/m2
represents a Pascal, the unit of pressure. This implies that the change of phase suffered by the
–
beam when the external pressure changes by 1 Pa over 1 m of the fiber is 3 × 10 4. When
–
someone whispers, the sound pressure corresponds to about 2 × 10 4 Pa at a distance of 1 m. If
the length of the sensing arm is 100 m, the corresponding phase change in the light propagating
–
through the sensing arm is 6 × 10 6 rad. Such small changes in phase are detectable by sensitive
signal processing.
MZ sensors can be used to sense different physical parameters such as temperature, strain, and
magnetic field. These physical parameters cause changes in the phase of the propagating light
beam. Such sensors are finding various applications in hydrophones for underwater sound
detection. One of the great advantages of such an application is the possibility of configuring
the sensors as omni-directional or highly directional sensors.
Fiber optic rotation sensor—the fiber optic gyroscope (FOG)—One of the more
important fiber optic sensors is the fiber optic gyroscope, capable of measuring rotation rate.
The FOG is a device with no moving parts, with improved lifetime, and of relatively low cost.
Thus FOGs are rapidly replacing conventional mechanical gyros for many applications.
The principle of operation of the fiber optic gyroscope is based on the Sagnac effect.
Figure 4-39 shows a simple FOG configuration. It consists of a loop of polarization maintaining,
single-mode optical fiber connected to a pair of 3-dB directional couplers (capable of splitting
the incoming light beam into two equal parts or combining the beams from both input fibers), a
polarized source, and a detector. Light from the source is split into two equal parts at the coupler
C1, one part traveling clockwise and the other counterclockwise in the fiber coil. After
traversing the coil the two light beams are recombined at the same coupler and the resulting
light energy is detected by a photodetector connected to the coupler C2. The source in a FOG is
usually a source with a broad spectrum and hence a short coherence length. The source is
chosen to avoid any coherent interference between backscattered light from the two counterpropagating beams in the fiber loop. This could be a superluminescent diode or a
superfluorescent fiber source.
Figure 4-39 A fiber optic gyroscope for rotation sensing based on the Sagnac effect
We first note that, if the loop is not rotating, the clockwise and the counterclockwise beams will
take the same time to traverse the loop and hence arrive at the same time with the same phase at
the coupler C1. On the other hand, when the loop begins to rotate, the times taken by the two
beams are different. This can be understood from the fact that, if the loop rotates clockwise, by
the time the beams traverse the loop the starting point will have moved and the clockwise beam
46
Optics and Photonics Series, Course 2: Elements of Photonics
will take a slightly longer time than the counterclockwise beam to come back to the starting
point. This difference of time or phase will result in a change of intensity at the output light
beam propagating toward C2.
One of the great advantages of a Sagnac interferometer is that the sensor gives no signal for
reciprocal stimuli, i.e., stimuli that act in an identical fashion on both the beams. Thus a change
of temperature affects both the beams (clockwise and counterclockwise) equally and so
produces no change in the output.
If the entire loop arrangement rotates with an angular velocity Ω, the phase difference
∆φ (radians) between the two beams is given by
∆φ =
8πNAΩ
cλ o
(4-29)
where N is the number of fiber turns in the loop,
A is the area enclosed by one turn (which need not be circular),
λο is the free space wavelength of light, and
c is the speed of light in a vacuum.
Example 14
Let us consider a fiber optic gyroscope with a coil of diameter 10 cm, having 1500 turns
(corresponding to a total fiber length of πDN ~ 470 m) and operating at 850 nm. The corresponding
phase difference, determined from Equation 4-29 is ∆φ= 1.16 Ω rad. If Ω corresponds to the rotation
rate of the Earth (15° per hour) the corresponding phase shift is ∆φ = 8.4 × 10–5 rad, a small shift
indeed. This phase difference corresponds to a flight time difference between the two beams of
∆τ =
∆φλ o
∆
∆φ
=
=
≈ 3.8 × 10–20 s.
ω
2πf o
2πc
There are many different ways of operating the gyroscope. One of them is called the closed-loop
operation. In this method, a pseudo rotation signal is generated in the gyro to cancel the actual
signal caused due to the rotation, thus nulling the output. This is achieved by having a phase
modulator near one end of the loop as shown in Figure 4-39. The counterclockwise-traveling
beam encounters the phase modulator later than the clockwise beam. This time difference
introduces an artificial phase difference between the two beams. The applied signal on the
modulator required to null the signal gives the rotation rate.
Fiber optic gyros capable of measuring from 0.001 deg/h to 100 deg/h are being made.
Applications include navigation of aircraft, spacecraft, missiles, manned and unmanned
platforms, antenna piloting and tracking, and a compass or north finder. Various applications
require FOGs with different sensitivities: Autos require about 10 to 100 deg/h, attitude reference
for airplanes requires 1 deg/h, and precision inertial navigation requires gyros with 0.01 to
0.001 deg/h. A Boeing 777 uses an inertial navigation system that has both ring laser
gyroscopes and FOGs.
Module 2-4: Principles of Fiber Optic Communication
47
An interesting application involves automobile navigation. The automobile gyro provides
information about the direction and distance traveled and the vehicle’s location, which is shown
on the monitor in the car. Thus the driver can navigate through a city. Luxury cars from Toyota
and Nissan sold in Japan have FOGs as part of their on-board navigation systems.
LABORATORY
Using the concepts developed in this module, you will be able to perform the following simple
experimental projects as part of the laboratory exercises for this module.
•
Measure the numerical aperture (N.A.) of a plastic multimode optical fiber.
•
Measure the attenuation coefficient of a plastic multimode optical fiber.
•
Construct a 2 × 2 fiber optic coupler.
•
Demonstrate wavelength-division multiplexing.
Equipment
Laser pointer
100-meter spool of 1-mm diameter plastic multimode optical fiber
1 razor blade
1 red and 1 greed LED
2 180-Ω resistors
1 5-volt power supply
1 plastic mounted diffraction grating (~15,000 lines/inch)
(A) N.A. of a multimode optical fiber
Cut off a one-meter segment of plastic fiber from the spool with a razor blade. Make sure that
both ends of the fiber are cut straight. Place the laser point up against one end of the fiber and
shine the light into the fiber. You should see the light coming out of the other end of the fiber.
Place a piece of white paper one foot (Z) from the end of the fiber such that the light generates a
spot on the paper. Draw a concentric circle on the paper to indicate the diameter of the spot as
illustrated in Figure 4-40. Measure the diameter of the spot with a ruler. This is D. The N.A. is
calculated using the following equation.
N.A. = sin θa = sin [tan–1 (D/2z)]
48
(4-30)
Optics and Photonics Series, Course 2: Elements of Photonics
Figure 4-40 Measurement of the diameter D of the spot on a screen placed at a far-field distance z
from the output end of a multimode fiber can be used to measure the N.A. of the fiber.
(B) Attenuation measurement
A simple experiment can be performed for measuring the attenuation of the fiber at one specific
wavelength. Cut a length L (about 1 km) of the fiber and couple the beam from a laser pointer
squarely into the fiber. Measure the power Po exiting at the output end of the fiber. Without
disturbing the coupling system, cut off a reference length of 1 m of the fiber from the input end.
Measure the power Pi exiting from the 1-m length of the fiber. This will be the input power to
the longer portion of the fiber. The attenuation coefficient α of the fiber at the wavelength of the
laser is then given by
α(dB/km) ≈
⎛P⎞
1
log ⎜ i ⎟
L
⎝ Po ⎠
(C) Making a fiber optic coupler
1. With the razor blade, carefully strip off approximately 3″ of the fiber jacket in the middle
of a 1-foot segment of fiber. (See Figure 4-41.) Repeat using another 1-foot segment of
fiber so that you have two identical pieces.
Figure 4-41
2. Where the fiber has been stripped, twist the two fibers together as shown n Figure 4-42.
3. On each end of the stripped area, place a small weight (e.g., paperweight, book) to hold
the fiber in place. (See Figure 4-42.)
Module 2-4: Principles of Fiber Optic Communication
49
Figure 4-42
4. Using the heat-shrink gun set on the low temperature setting, apply heat to the twisted
area. Move the heat gun gently back and forth to uniformly melt the fiber. CAUTION: Do
not hold the heat gun stationary because the fiber will melt quickly!
5. As the fiber is heated, you will notice that it will contract a bit. This is normal. When the
contraction subsides, remove the heat gun and let the fiber cool for a minute.
6. Shine the laser pointer into port 1 on wire 1 of the coupler. You should observe a fair
amount of coupling (~20–30%) into port 3 of wire 2 of the coupler. (See Figure 4-42.) If
more coupling is needed, repeat the heating process until the desired coupling is obtained.
Without disturbing the pair of twisted wires, use them in the next procedure in a WDM
demonstration.
(D) Wavelength-division multiplexing demonstration
1. Using an electronics breadboard, connect two LEDs (1 red, 1 green) as shown in
Figure 4-43. Position the LEDs (bend the leads) so that the top of the LED is positioned
horizontally. Make sure the LEDs are lit brightly.
2. Using black electrical tape, connect input ports 1 and 4 of Figure 4-43 to the tops of the
red and green LEDs respectively.
Figure 4-43
3. Now observe the output at port 2 of Figure 4-42. The red and green colors will be mixed.
50
Optics and Photonics Series, Course 2: Elements of Photonics
4. To separate the colors, observe the output of port 2 through a diffraction grating. You
should observe a central bright spot (coming from the fiber) and two identical diffraction
patterns—one on either side—with the red and the green separated. (See Figure 4-44.) To
ensure that the two signals are indeed independent, turn of the LEDs one at a time and
observe the output of port 2 through the diffraction grating.
Figure 4-44
PROBLEMS
1. A fiber of 1-km length has Pin = 1 mW and Pout = 0.125 mW. Find the loss in dB/km.
2. The power of a 2-mW laser beam decreases to 15 µW after the beam traverses through
25 km of a single-mode optical fiber. Calculate the attenuation of the fiber. (Answer:
0.85 dB/km)
3. A communication system uses 8 km of fiber that has a 0.8-dB/km loss characteristic. Find
the output power if the input power is 20 mW.
4. A 5-km fiber optic system has an input power of 1 mW and a loss characteristic of
1.5 dB/km. Determine the output power.
5. What is the maximum core diameter for a fiber to operate in single mode at a wavelength
of 1310 nm if the N.A. is 0.12?
6. A 1-km-length multimode fiber has a modal dispersion of 0.50 ns/km and a chromatic
dispersion of 50 ps/km • nm. If it is used with an LED with a linewidth of 30 nm, (a) what
is the total dispersion? (b) What is the bandwidth (BW) of the fiber?
7. A receiver has a sensitivity Ps of – 40 dBm for a BER of 10–9. What is the minimum power
(in watts) that must be incident on the detector?
Module 2-4: Principles of Fiber Optic Communication
51
8. A system has the following characteristics:
• LD power (PL) = 1 mW (0 dBm)
• LD to fiber loss (Lsf) = 3 dB
• Fiber loss per km (FL) = 0.2 dB/km
• Fiber length (L) = 100 km
• Connector loss (Lconn) = 3 dB (3 connectors spaced 25 km apart with 1 dB of loss each)
• Fiber to detector loss (Lfd) = 1 dB
• Receiver sensitivity (Ps) = – 40 dBm
Find the loss margin and sketch the power budget curve.
9. A 5-km fiber with a BW × length product of 1200 MHz × km (optical bandwidth) is used
in a communication system. The rise times of the other components are ttc = 5 ns, tL = 1
ns, tph = 1.5 ns, and trc = 5 ns. Calculate the electrical BW for the system.
10. A 4 × 4 star coupler is used in a fiber optic system to connect the signal from one
computer to four terminals. If the power at an input fiber to the star coupler is 1 mW, find
(a) the power at each output fiber and (b) the power division in decibels.
11. An 8 × 8 star coupler is used to distribute the +3-dBm power of a laser diode to 8 fibers.
The excess loss (Lossex) of the coupler is 1 dB. Find the power at each output fiber in
dBm and µW.
REFERENCES
Bennet, S. “Fibre Optic Gyro System Keeps Bus Riders Informed,” Photonics Spectra,
August 1996, pp. 117–120.
Burns, W.K. “Fiber Optic Gyroscopes—Light Is Better,” Optics and Photonics News,
May 1998, pp. 28–32.
Chynoweth, A.G. “Lightwave Communications: The Fiber Lightguide,” Physics Today, 29 (5),
28, 1976.
Farmer, K.R., and T.G. Digges. “A Miniature Fiber Sensor,” Photonics Spectra, August 1996,
pp. 128–129.
“Fiber Optic Technology Put to Work—Big Time,” Photonics Spectra, August 1994, p. 114.
Gambling, W.A. “Glass, Light, and the Information Revolution, Ninth W.E.S. Turner Memorial
Lecture,” Glass Technology, Vol. 27 (6), 179, 1986.
Ghatak, A., I.C. Goyal, and R. Varshney. Fiber Optica: A Software for Characterizing Fiber
and Integrated Optic Waveguides. New Delhi: Viva Books, 1999.
52
Optics and Photonics Series, Course 2: Elements of Photonics
Ghatak, A., and K. Thyagarajan. Introduction to Fiber Optics. Cambridge: Cambridge
University Press, 1998.
Grifford, R.S., and D.J. Bartnik. “Using Optical Sensors to Measure Arterial Blood Gases,”
Optics and Photonics News, March 1998, pp. 27–32.
Hotate, K. “Fiber Optic Gyros,” Photonics Spectra, April 1997, p. 108.
Ishigure, T., E. Nihei, and Y. Koike. “Optimum Refractive Index Profile of the Graded Index
Polymer Optical Fiber, Toward Gigabit Data Links,” Applied Optics, Vol. 35, 1996,
pp. 2048–2053.
Kao, C.K., and G.A. Hockham. “Dielectric-Fibre Surface Waveguides for Optical Frequencies,”
Proc. IEEE, Vol. 113 (7), 1151, 1966.
Kapron, F.P., D.B. Keck, and R.D. Maurer. “Radiation Losses in Glass Optical Waveguides,”
Applied Physics Letters, Vol. 17, 1970, p. 423.
Katzir, A. “Optical Fibers in Medicine,” Scientific American, May 1989, pp. 86–91.
Keiser, G. Optical Fiber Communications. New York: McGraw Hill, 1991.
Koeppen, C., R.F. Shi, W.D. Chen, and A.F. Garito. “Properties of Plastic Optical Fibers,”
Journal of the Optical Society of America, B Vol. 15, 1998, pp. 727–739.
Koike, Y., T. Ishigure, and E. Nihei. “High bandwidth graded index polymer optical fiber,”
IEEE Journal of Lightwave Technology, Vol. 13, 1995, pp. 1475–1489.
Maclean, D.J.H. Optical Line Systems. Chichester: John Wiley, 1996.
Marcou, J., M. Robiette, and J. Bulabois. Plastic Optical Fibers, Chichester: John Wiley and
Sons, 1997.
Marcuse, D. “Loss Analysis of Single Mode Fiber Splices,” Bell Systems Tech. Journal,
Vol. 56, 1977, p. 703.
Miya, T., Y. Terunama, T. Hosaka, and T. Miyashita. “An Ultimate Low Loss Single Mode
Fiber at 1.55 mm,” Electron. Letts., Vol. 15, 1979, p. 106.
“Schott Is Lighting the Way Home,” Fiberoptic Product News, February 1997, p. 13.
Spillman, W.B., and R.L. Gravel. “Moving Fiber Optic Hydrophone,” Optics Letters, Vol. 5,
1980, pp. 30–33.
Module 2-4: Principles of Fiber Optic Communication
53